This invention pertains generally to a stabilizing suspension system for a vehicle capable of travelling over rough terrain. More particularly, the invention pertains to a passive stabilizing suspension system for use in vehicles carrying telescopic booms or aerial work platforms. These vehicles are generally intended for elevated use only on firm level ground, otherwise the vehicle may overturn. However, in order to move the vehicle into position, it is desirable that the vehicle have the capability of travelling over rough terrain, and to be able to move on rough terrain while elevated.
The prior art includes the Barnhart U.S. Pat. No. 5,447,331 dated Sep. 5, 1995, which teaches an active stabilization system. That patent discloses a sensor which is activated by oscillation of the rear axle. The sensor controls an active mechanism which extends a hydraulic cylinder on the front axle, thereby exerting enough force to raise one front corner of the chassis. The Barnhart patent requires that both the front and rear axle oscillate, although the rear axle has limited oscillation, but the freedom of both axles to oscillate can result in a rocking of the chassis and superstructure. The system of the Barnhart patent may, therefore, tend to bounce or rock back and forth between stops provided on the rear axle. Another disadvantage of the Barnhart patent is that it teaches an active suspension, that is the fluid is actively pumped to the hydraulic cylinder on the lower side in order to achieve sufficient force to raise the chassis. Such an active system is relatively complicated, relatively expensive to manufacture and somewhat more subject to malfunction.
The prior art also includes the Nakagawa U.S. Pat. No. 4,382,604 dated May 10, 1983. That patent teaches the use of an oscillating axle with two hydraulic cylinders disposed between the axle and the chassis. When the operator parks the vehicle and applies the brakes, the cylinders are filled with fluid to make the axle act as a fixed axle. However, a major disadvantage of Nakagawa is that the stiffening of the cylinders to resist tipping only occurs when the vehicle is parked.